1. Field of the Invention
The present invention relates to a hydrogen storage alloy which can electrochemically absorb and desorb hydrogen in a reversible manner, and it also relates to an electrode using the hydrogen storage alloy.
2. Description of the Prior Art
Storage batteries, which are widely used as a variety of power sources, have two general classifications: lead and alkaline. Compared with lead storage batteries, alkaline storage batteries have higher reliability and can be made smaller in size, so that they are used in various kinds of portable electric devices. On the other hand, large-sized alkaline storage batteries are widely used for industrial equipment.
Some alkaline storage batteries use, for example, air or silver oxide for their positive electrodes. But in most cases, the positive electrodes are made of nickel. The positive electrodes of alkaline storage batteries have previously been of a pocket type, but at present, those of a sintered type are generally used. This change in the configuration of the positive electrodes has improved the characteristics of the alkaline storage batteries. Furthermore, it has become possible to hermetically seal the batteries, so that the alkaline storage batteries have come to find wider application.
As the negative electrodes of alkaline storage batteries, cadmium electrodes are generally used. Other materials for the negative electrodes include zinc, iron, hydrogen, and the like.
The cadmium electrodes, however, have a small theoretical energy capacity. Zinc electrodes also have disadvantages in that they are susceptible to deformation and that dendrites tend to be formed therein.
In order to attain a larger energy capacity, an electrode made of a metal such as a hydrogen storage alloy or a hydride thereof has been developed to be used in, for example, a nickel-hydrogen storage battery. Many methods for the production of such a nickel-hydrogen storage battery have been proposed.
A hydrogen storage alloy such as mentioned above can absorb and desorb hydrogen through a reversible electrochemical reaction. When the negative electrode of an alkaline storage battery is made of this hydrogen storage alloy, it attains a larger theoretical energy capacity, as compared with the cadmium electrodes. Furthermore, unlike the zinc electrodes, when using the hydrogen storage alloy electrode, neither deformation of the electrode nor dendrite-growth in the electrode occurs. Therefore, it is expected that hydrogen storage alloys will provide electrodes having a large energy capacity and a longer lifetime with no possibility of causing pollution.
Examples of well known hydrogen storage alloys to be used for electrodes include multi-element alloys such as those of the Ti-Ni system, the La(or Mm)-Ni system (Mm is a misch metal), and the like.
The Ti-Ni alloys are of an AB type. When such a Ti-Ni alloy is used for the negative electrode of an alkaline storage battery, the electrode exhibits relatively large discharging capacities in the early charging and discharging cycles. With the repetition of charging and discharging, however, it becomes difficult to keep the discharging capacity from decreasing undesirably.
The La(or Mm)-Ni alloys are known as AB.sub.5 -type multi-element alloys. Many attempts have recently been made to develop alloys of this type because they have been considered as suitable materials for electrodes. However, an electrode using the La(or Mm)-Ni alloy has a relatively small discharging capacity and a short lifetime. Furthermore, materials for the La(or Mm)-Ni alloys are expensive.
Another hydrogen storage alloy which has been developed is a Laves phase alloy of an AB.sub.2 type (where A is Zr, Ti, or the like, which has a high affinity for hydrogen, and B is Ni, Mn, Cr or one of other transition elements). The Laves phase alloys have a large capacity for absorbing hydrogen, and electrodes using these alloys have a large energy capacity and a long lifetime. As examples of the Laves phase alloys, there are an alloy of the AB.sub.a system (U.S. Pat. No. 4,946,646), an alloy of the Zr.alpha.V.beta.Ni.gamma.M.delta. system (Japanese Laid-open Patent Publication No. 64-60961), an alloy of the AxByNiz system (Japanese Laid-open Patent Publication No. 1-102855), and the like.
Electrodes using the above-mentioned conventional Laves phase alloys have a long lifetime and a large discharging capacity, as compared with electrodes using the multi-element alloys such as Ti-Ni and La(or Mm)-Ni alloys. However, the conventional Laves phase alloy electrodes have poor discharging characteristics in the early charging and discharging cycles.
A Laves phase alloy of the Zr-Mn-V-Cr-Ni system is known as another hydrogen storage alloy, but it also has the following disadvantage. When an electrode using this alloy is included in an alkaline storage battery, it cannot efficiently absorb or desorb a large amount of hydrogen in the early cycles of its electrochemical charging and discharging.
Thus, there has been a great demand for a novel hydrogen storage alloy which can be used to produce an electrode having a large energy capacity and a long lifetime and exhibiting excellent discharging characteristics in the early charging and discharging cycles.